I am actually going through something very similar...it is hard to read people...especialy family members when it comes to money matters because it is such a touchy subject. My advice to you is to sit down with your father and have a honest conversation. ask him why your sister didn't have to save a dime and you do or maybe in a nice way say that you understand his decision...but you need to know exactly how much he is expecting to pay and tell him that prices jump for venues every year so yes your wedding will be a bit more expensive. just keep your cool,be honest and keep the lines of communication open!

PS thank you for your advice! it helped me out a lot!